Equipment, containers, packaging and foodstuffs provide surfaces for the accumulation of detritus and surfaces for microorganism colonisation and growth. This accumulation of detritus and microorganism growth can cause fouling and reduce the efficiency of the equipment, the quality of the product produced using that equipment and reduce the life of equipment, containers and packaging. Furthermore, microorganism growth leads to premature spoilage of products, particularly foodstuffs or cross-contamination with micro organisms causing food borne illness. Microorganism biofilms resistant to inadequate nutrient supply, drying, adverse temperature, abrasion or chemicals may form on surfaces of foodstuffs, containers or equipment such as condensors, heat exchangers, valves, pipes, vessels, air cooling towers or any surface exposed to moisture. Such contamination fouling or biofilms lead to spoilage of the foodstuffs, micro-organisms causing food borne illness or fouling of the containers or equipment.
Typically, spoilage is delayed by use of packaging materials, hygienic processing to reduce the load of spoilage organisms and refrigeration. However, these methods do not actively remove spoilage organisms. In addition, conventional washing processes do not remove microorganisms within a surface or adequately remove detritus tightly bound to a surface.
Contaminating microorganisms, biofilms and/or detritus are typically reduced using any one of a number of methods including washing, chemical treatments or physical removal. Washing with low or high pressure (680 to 2684 kPa), cold or warm water (60 to 82° C.) removes soft, but not hard deposits and provides limited surface disinfection. Steam cleaning is more efficient but will not disinfect the surface layers to the same depth that microorganism growth occurs and is not suitable for foodstuffs. Poor thermal conductivity through detritus inhibits heat transfer and thus microorganism elimination.
Chemical cleaning agents may dissolve surface detritus during cleaning although neutralising washes after such treatment is required. However, such chemicals have poor mass transfer effect through solid detritus and into surface layers of containers or other structures including fruits and vegetables. Thus, these methods result in poor reduction of microorganism load. Physical methods of cleaning and surface disinfection such as shaving, dry ice particle bombardment merely treat the surface and do not remove microorganisms deeper into the structure. Harsh physical methods and are not applicable to foodstuffs.
Conventional ultrasonic cleaning apparatus and methods have been utilised to clean a wide variety of material, including containers. However, the ultrasonic energy produced in a conventional apparatus creates standing waves so that the pattern of cleaning results in alternating partially cleaned zones in areas not bounded by the standing waves and uncleaned zones in the regions bounded by the standing waves. Furthermore, ultrasonic energy produced in a conventional apparatus does not penetrate into a surface and propagates only for a very short distance. In order to clean an article it must be moved relative to the standing wave which can be impractical for large articles.
Accordingly there exists a need in the art for apparatus and methods for improved cleaning and/or disinfection of surfaces.